The present invention relates to converging optical systems for a laser beam source, such as an optical system for an optical head for forming a very small beam spot by converging a laser beam emitted from a semiconductor laser with a lens system.
It is well known in the art that the beam intensity distribution of a laser beam emitted from a semiconductor layer, is approximated by the Gauss distribution and also that, by converging such a semiconductor laser beam to a very small beam spot with an objective lens having a substantially circular lens aperture, the converged beam spot varies in diameter in dependence on the beam intensity distribution in the lens aperture. More specifically, the smaller spot diameter is obtainable the more uniform is the beam intensity distribution in the lens aperture, that is, the smaller the beam intensity difference between the center and the edge of the lens aperture. The semiconductor laser serves as laser beam source for providing a divergent beam output with the beam emission position as substantial point beam source, and it has a character that its beam emission angle is different in two directions perpendicular to the optical axis of the emitted laser beam. FIG. 2 shows beam intensity distributions plotted against the beam emission angle from two perpendicular directions, i.e., a U axis and a V axis direction. It will be seen that for the same beam intensity the beam emission angle from the U axis direction is smaller. The beam emission angle is often expressed in terms of angle corresponding to one half the maximum beam intensity on the optical axis. As shown by arrow mark in the Figure, the angle from the U axis direction is smaller.
Thus, it will be seen that by converging a laser beam with different beam emission angles in the two directions as shown in FIG. 2A as a very small spot in a circular objective lens aperture, a large diameter converged beam spot, i.e., an oval spot, is produced because of a large beam intensity distribution difference of the spot dimension in the U axis direction from the spot dimension in the V axis direction. This phenomenon is undesired when it is desired to utilize as circular converged beam spot as possible, for instance in the case of an optical head for an optical disc.
As a simple method to be used for avoiding the production of such an oval converged beam spot and obtaining as small spot diameter as possible, is one in which the lens aperture corresponding to the beam emission angle is reduced to obtain beam convergence of only the neighborhood of the beam intensity distribution center. This means that the distance between the lens and the laser beam source is increased when the lens aperture diameter is the same, that is, a long focal distance lens is used. This method, however, has a problem that the optical system is increased in size as a whole because the distance between the lens and the laser beam source is increased.
FIG. 10 shows a different prior art technique, in which the beam intensity distribution of a laser beam is enlarged in a particular direction. In this example, a laser beam emitted from a laser beam source 1 is collimated in a collimator lens to a parallel beam. A triangular shaping prism 8 enlarges the beam intensity distribution of the parallel beam in the U axis direction. An objective lens 3 converges the output beam from the prism 8. Since in this case the beam intensity distribution is enlarged only in the small beam emission angle direction, it is possible to obtain a very small beam spot without substantially deteriorating the beam utilization factor. This method, however, requires a separate component for shaping the beam and also place of disposition of the component. In addition, since the diffraction by the prism is utilized, a problem arises that a change in the wavelength of the emitted laser beam results in a change in the direction of the emitted laser beam.
An object of the present invention is to provide a converging optical system, which is free from the problems discussed above and permits formation of a very small beam spot relatively easily.
According to an aspect of the present invention, there is provided a converging optical system for a laser bean source comprising a laser beam source for emitting a laser beam in a substantially Gauss intensity distribution and with a beam emission pattern with different beam emission angles in perpendicular directions, an optical system including an objective lens for receiving a laser beam in a portion of the beam emission intensity distribution in the neighborhood of optical axis in the aperture and converging the received laser beam to a very small spot, and a beam intensity filter with increasing permeability with increasing distance from the optical axis in a particular direction, the beam intensity filter being disposed in the scope of laser beam emitted from the laser beam source such that the particular direction is coincident with a small beam emission angle direction.
According to another aspect of the present invention, there is provided a converging optical system for a laser beam source comprising a laser beam source for emitting a laser beam in a substantially Gauss intensity distribution and with a beam emission pattern with different beam emission angles in perpendicular directions, an optical system including an objective lens for receiving a laser beam in a portion of the beam emission intensity distribution in the neighborhood of optical axis in the aperture and converging the received laser beam to a very small spot, and a beam intensity filter with increasing permeability with increasing distance from the optical axis in a particular direction, the beam intensity filter being disposed in the scope of laser beam emitted from the laser beam source such that the particular direction is coincident with a small beam emission angle direction and the output beam through the beam intensity filter of the laser beam source being such that the beam emission angle in the particular direction and the beam emission angle in the perpendicular direction are substantially equal.
The converging optical system satisfies a relation
Y less than xe2x88x920.14xc3x97X+0.68
where X represents the ratio between the beam emission angles in large and small beam emission angle directions in the emitted beam intensity distribution, and Y represents the ratio of the dose of laser beam transmitted through the beam intensity filter and received in the aperture of the objective lens to the dose of laser beam emitted from the laser beam source.
The beam intensity filter is constructed such that its permeability increases in a straight fashion, a curved fashion or a staircase fashion as one goes from the optical axis oppositely in the particular direction.
The beam intensity filter is constructed as a diffraction grating, non-diffracted laser beam component from the diffraction grating being utilized as beam intensity filter output.
The converging optical system further comprises a beam-separating optical system for separating a beam reflected from the point of convergence of laser beam converged by the objective lens from the laser beam emitted from the laser beam source and leading the separated beam to an optical detector, the beam-separating optical system being disposed between the objective lens and the beam intensity filter.
According to other aspect of the present invention, there is provided an optical system for converging a laser beam of a substantially Gauss intensity distribution and with a beam emission pattern with different beam emission angles in perpendicular directions emitted from a laser beam source through a beam intensity filter having a sectional transmitted beam intensity (permeability) distribution and an objective lens, wherein the beam intensity filter has the sectional transmitted beam intensity (permeability) distribution with increasing permeability with increasing distance from the optical axis in a particular direction, and is disposed in the scope of laser beam emitted from the laser beam source such that the particular direction is coincident with a small beam emission angle direction.
Other objects and features will be clarified from the following description with reference to attached drawings.